1. Field of the Invention
The present invention relates to a circuit and method for a switching power supply system. Particularly, the invention relates to a circuit and a method for controlling values, such as voltage and current, of the electric power outputted from a main unit of a switching power supply system by controlling the duty ratio in the ON/OFF operation of a switching device, in such a manner that it reduces internal current consumption during times of light or no load so as to increase the output conversion efficiency of the switching power supply.
2. Description of the Background Art
FIG. 5 is a block diagram showing an arrangement of a current ordinary switching power supply system. Such a switching power supply system is formed with a main unit and a control block.
The main unit converts the inputted electric power to an output. A DC-DC converter 92 is used for the main unit in the arrangement of FIG. 5 (although various kinds of devices including components such as inverters and converters also could be used for the main unit).
The control block, which is a control circuit 91 for the system shown in FIG. 5, controls the main unit. The control block 91 includes a duty ratio control circuit 911, a logic circuit 912 and a protection circuit 913. The control block 91 also includes analog circuits such as a reference voltage circuit and a bias circuit and other logic circuits. These latter circuits, however, have no direct connection to an explanation of the operation of the switching power supply system here, and the explanations thereof are omitted.
In the switching power supply system shown in FIG. 5, the output voltage VOUT of the DC-DC converter 92 is fed back to the duty ratio control circuit 911 in the control block 91. The duty ratio control circuit 911 outputs a pulsed signal VCONT with its duty ratio being controlled according to the error between the output voltage VOUT and a target output voltage. The duty ratio is a ratio of the length of time that the level of the outputted pulse is at HIGH (a voltage value at a high level in a binary form of a low level voltage value and a high level voltage value) or at LOW (a voltage value at a low level in the binary form of the low level voltage value and the high level voltage value) to the period of the pulse. Furthermore, the pulsed signal VCONT, with its duty ratio being controlled, performs ON/OFF control of switching devices provided in the DC-DC converter 92, such as semiconductor switching devices represented by bipolar transistors and MOSFETs or mechanical switching devices represented by relay circuits. The output voltage VOUT is controlled by this ON/OFF control.
Moreover, by means of an external startup/shutdown signal, startup/shutdown of the switching power supply system can generally be carried out. In the system shown in FIG. 5, by externally applying a startup/shutdown signal VE1 to the control block 91, its startup (and operation)/shutdown is carried out.
In order to protect the power supply system, the protection circuit 913 includes a circuit that monitor an inputted voltage VIN, an input current, the temperature of a power supply, an output voltage or an output current. When the presence of a specified abnormal state is detected using one of these kinds of monitoring, the protection circuit 913 outputs a power supply shutdown signal VE2 to lead the power supply to a shutdown.
The logic circuit 912 produces a signal VE3 on the basis of the startup/shutdown signal VE1 and the power supply shutdown signal VE2, to output the signal VE3. When the startup/shutdown signal VE1 is a signal indicating starting-up, and the power supply shutdown signal VE2 is a signal indicating a normal state, the signal VE3 is outputted as a signal for starting up the power supply. When the startup/shutdown signal VE1 is a signal indicating shutdown or the power supply shutdown signal VE2 is a signal indicating the presence of an abnormal state, the signal VE3 is outputted as a power supply shutdown signal for shutting down the power supply system.
FIG. 6 to FIG. 13 illustrate examples of possible circuit configurations of the DC-DC converter 92 in the current switching power supply system shown in FIG. 5. In each of the examples, a switch S1 shown in each of the figures and a switch S2 shown in each of FIGS. 7, 9, 11 and 13 are subjected to ON/OFF control by the duty ratio control pulsed signal VCONT outputted from the duty ratio control circuit 911 in the control block 91.
Letting the duty ratio of the switch S1, that is the ratio of the duration of the on-state of the switch S1 to its switching period, be D1 and the duty ratio of the switch S2, that is the ratio of the duration of the on-state of the switch S2 or a diode D2 to its switching period, be D2, the output voltage VOUT is calculated by the following expressions when the internal loss of the power supply is neglected.
In the circuits shown in FIG. 6 and FIG. 7, which use a down-converter, the output voltage VOUT is calculated by the expression VOUT=VIN*D1/(D1+D2).
In the circuits shown in FIG. 8 and FIG. 9 which use an up-converter, the output voltage VOUT is calculated by the expression VOUT=VIN*(D1+D2)/D2.
In the circuits shown in FIG. 10 to FIG. 13, which use a buck-boost converter or a flyback converter, the output voltage VOUT is calculated by the expression VOUT=VIN*D1/D2.
By adjusting the duty ratio of the duty ratio controlled pulsed signal VCONT outputted from the duty ratio control circuit 911 in the control block 91, the power attribute value (here, the voltage value) of the output voltage is controlled.
In current switching power supply systems provided with a DC-DC converter such as those shown in each of FIG. 6 to FIG. 13, the amount of switching losses, such as those due to parasitic resistance within the power supply and losses due to power used to drive the control circuit, increase to thereby reduce output conversion efficiency.
Moreover, in a battery-powered device such as a portable cell phone, when the device is in a stand-by state, components such as a CPU (Central Processing Unit) and a DSP (Digital Signal Processor) as loads of a switching power supply are sometimes shut down to extend the life of the battery. In this case, the switching power supply system is brought into a no-load state. Nevertheless, even though loads (those such as a CPU and a DSP) are made to shut down to reduce their driving losses, a loss accompanying switching of the switching power supply system and a loss due in driving the control circuit are still produced.
Thus, for reducing current consumption under no-load or light-load conditions, the following approach is widely taken. Control with PWM (Pulse Width Modulation) with a fixed switching period is carried out at heavy load and, at no-load or light load, PFM (Pulse Frequency Modulation) or an operation of stopping the switching operation for a certain duration (i.e. an intermittent operation) is carried out, thereby to make the average period of the switching longer than that at heavy load. This reduces the switching loss at no-load or light load. Also, if PFM control is adopted for loads ranging from light to heavy, the average period of the switching becomes longer than that at heavy load. Thus, the switching loss at no-load or light load can be reduced.
With this approach, however, although the loss accompanying switching is reduced, driving losses in analog circuits in the control circuit and in circuits independent of switching operations cannot be reduced. In particular, in the protection circuit, current consumption is in the range of tens to hundreds of microamperes. The power loss due to current consumption is a main loss in the control circuit in a no-load state. At present, a portable electronic device such as a cellular phone is required to reduce current consumption at no-load in its switching power supply system down to the tens of microamperes range or less.
For example, Japanese patent publication number JP-A-2003-284241, proposes that protection circuits be rendered so as not to operate in a voltage regulator and a DC-DC converter when their respective load current values are less or equal to specified values. With the method proposed in JP-A-2003-284241, although low current consumption can be achieved, there is a problem that the presence or absence of an abnormal state cannot be monitored at no-load or light load. The invention was made in view of the above-described problems.